The granular material made of crushed basalt was adopted, where it was uniformly graded between the particle sizes of 5 mm−10 mm, as shown in Figure 1. Its minimum and maximum void ratios were measured to be 0.60 and 0.89, respectively. The permeability coefficient of the gravel was 2.26 cm/s. To investigate the effect of clogging on the permeability behaviour of PPGM, a sand with the gradation shown in Figure 1 was also adopted, where it had a coefficient of uniformity of 3.47, a maximum particle size of 5 mm and a minimum particle size of 0.075 mm. In this experiment, non-foamed polyurethane was used, which mainly consists of two components: component A, the adhesive, and component B, the curing agent, which is prone to oxidation reactions in the air. The polyurethane polymer shown in Figure 2A was produced by mixing the polyol (Figure 2B) and polymethylene polyphenyl isocyanate (Figure 2C), based on a volume ratio of 3:2. Then, the polyurethane polymer was poured into the granular material, based on the contents of 3%, 4%, 5% and 6%, respectively, to produce each PPGM sample tested in this study. The detailed sample preparation steps are as follows: (1) Wash the aggregate thoroughly to remove any surface mud, then allow it to dry; (2) Measure the required amounts of aggregate and polyurethane components A and B according to the design mix and sample volume; (3) Mix polyurethane components A and B thoroughly for about 1 min until they are completely blended; (4) Gradually pour the mixed polyurethane in batches onto the aggregate while stirring, and stop mixing once the surface of the mixture is completely coated with the adhesive; (5) Pack the mixture into the mold in two layers, tamping each layer 15 times, and level the top surface. Once the sample reaches a certain strength (generally after 1 day), demold the sample.

The porosity of the PPGM samples with different contents ( f c ) of polyurethane polymer were tested by volume-based method. The net weight ( m 1 ) of a PPGM sample was firstly measured; then, it was submerged in into the water and saturated until the air was discharged, after which its weight ( m 2 ) in water was measured again. The porosity ( n ) of the sample as then calculated as in Equation 1: n = 1 − m 1 − m 2 V ρ w (1) where V and ρ w are the volume of the sample and the density of water, respectively. Table 1 lists the porosity of each sample.

The permeability of each PPGM sample was tested under constant water head. As the effect of clogging on the permeability of PPGM should be considered, a modified permeability test instrument instead of the traditional one [15], as schematically shown in Figure 3, was used. The samples were placed in a cylindrical PVC pipe with a diameter of 7.5 cm and a height of 20 cm. Due to the open channels of PPGM connected to the side wall of the test instrument, the water can easily flow through these channels along the side wall, resulting in an inaccurate measurement of the permeability coefficient ( k ) of PPGM. To resolve this limitation, all the samples were wrapped with the PE plastic wrapper before being placed into the test instrument. To prevent further leakage along the side wall, the contact surface between the wrapper and the side wall of the test instrument was also sealed with vaseline. Then, the tests under constant water head were carried out on PPGM without clogging. The effect of clogging on the permeability of PPGM was carried out by progressively adding sand on the top of the PPGM in the cylindrical PVC pipe. Firstly, the valve was closed and the water level was kept at the target value shown in Figure 3; Secondly, 5 g sand was evenly added to the top of the sample and rested for 1 min; thirdly, switch on the valve, let water and sand particles infiltrate through the PPGM under constant water level; fourthly, measure the permeability and keep record when the data of permeability is stable; repeat the above process for 35 times ( N ) and stop the test. To evaluate the effect of clogging, an index, called the remaining permeability ( S ) which measures the remaining capacity for water permeability, is used, as shown in Equation 2: S = k i k 0 (2) where k 0 is the coefficient of permeability (cm/s) of clean PPGM, k i is the coefficient of permeability (cm/s) of clogged PPGM after i -th addition of sand.

Figure 4 shows the coefficient of permeability ( k ) of PPGM without clogging. It can be found that k increases with an increase of the porosity while it decreases with an increase in the content of polyurethane polymer. This can be attributed to the block or size reduction of the open channels of PPGM caused by increasing the content of polyurethane polymer, which hinders the flow of water through PPGM.

Figure 5 shows the variation of the permeability of clogged PPGM after adding sand. It can be found that the permeability of PPGM decreases after adding sand. As the amount of sand increases, the extent of clogging increases which results in the reduction of the remain permeability, S . Three stages of the reduction of S can be observed in Figure 5. At first stage, the sand particles infiltrated through the open channels within PPGM and gradually blocked the channels with small constriction size, but most of the channels were still open and the reductions of S is moderate. With the further development of clogging, the permeability of PPGM reached the second stage, where sand particles blocked the majority of flowing channels with different constriction sizes, resulting in a rapid reduction of the permeability and the associated S . With the addition of more sand, the permeability of PPGM became stable and the third stage was reached. At this stage, the infiltration of sand led to a maximum clogging of PPGM. Table 1 also lists the value of S at the end of each test, i.e., S 35 . A remarkable reduction of the permeability of PPGM induced by clogging was observed. In the experiment, with the addition of sand into the water, some of the sand entered the internal pores of the specimen along with the water flow, while some accumulated on the surface of the specimen. As the amount of sand gradually increased, the accumulation of sand on the specimen’s surface also increased. Initially, a small amount of sand accumulated, followed by the majority being covered by a sand layer, leaving only the larger constriction size pores as main permeable channels. Eventually, the entire surface of the specimen was covered by the sand layer. In this experiment, as the amount of sand clogging increased, the permeability coefficient of PPGM with different polyurethane contents (porosity) consistently decreased, and eventually stabilized around 0.37 cm/s.

To further test the permeability of PPGM after clogging, the sand layers at the surface of the PPGM sample were removed. In this case, the loss of permeability was changed from around 16%–21% to around 75%–80%, as shown in Table 1, where S c indicates the remaining permeability after removing the sand layers at sample surface. This is because the permeability from previous tests takes into account the performances of both the clogged PPGM and the upper sand particles. After removing the cumulated fine particles from the surface of PPGM, the permeability can be partially recovered.